Blow molding is a generally known process for molding a preform part into a desired product. The preform is in the general shape of a tube with an opening at one end for the introduction of pressurized gas, typically air; however, other gases may be used. One specific type of blow molding is stretch blow molding (SBM). In typical SBM applications, a valve block provides both low and high-pressure gas to expand the preform into a mold cavity. The mold cavity comprises the outer shape of the desired product. SBM can be used in a wide variety of applications; however, one of the most widely used applications is in the production of Polyethylene terephthalate (PET) products, such as drinking bottles. Typically, the SBM process uses a low-pressure fluid supply along with a stretch rod that is inserted into the preform to stretch the preform in a longitudinal direction and radially outward and then uses a high-pressure fluid supply to expand the preform into the mold cavity. The low-pressure fluid supply along with the stretch rod is typically referred to as a pre-blowing phase of the molding cycle. The high-pressure fluid supply that expands the preform into the mold cavity is typically referred to as the blowing phase of the molding cycle. The low-pressure and high-pressure fluid supplies can be controlled using blow-mold valves. The resulting product is generally hollow with an exterior shape conforming to the shape of the mold cavity. The gas in the preform is then exhausted through one or more exhaust valves. This process is repeated during each stretch blow molding cycle.
FIG. 1 shows a prior art stretch blow molding valve block assembly 100. The prior art stretch blow molding valve block assembly 100 includes a valve block 102, a stretch rod 104, control chambers 106a-106d, operating chamber rings 108a-108d, valve pistons 110a-110d, and pilot valves 112. The stretch rod 104 extends vertically through the central chamber 101 and out the bottom of the valve block 102. The valve block 102 includes four sets of valves that are vertically stacked in the central chamber 101 and around the stretch rod 104. For example, the four sets of valves may correspond to a pre-blowing valve, a blowing valve, an air recovery valve, and an exhaust valve. As can be appreciated, a pilot air supply is provided by the pilot valves 112 in order to control the position of each valve piston 110a-110d. As can be seen, the valve pistons 110a and 110b are shown in the open position with the valve pistons 110c and 110d in the closed position. The valve block 102 also includes a number of inlet and outlet ports 114, 116, and 118. In use, the valve pistons are controlled using the various pilot valves 112 in order to direct the flow of pressurized gas through the valve block 102. In addition to the four valves shown, at least one additional valve or an electric motor is required to control the position of the stretch rod 104.
Typically, the blow molding process begins with a pre-blowing phase. During this phase, a pressure up to approximately 12 bar (174 psi) is provided to the preform while the stretch rod 104 simultaneously extends the preform in a longitudinal direction. During this pre-blowing phase, there is an attempt to substantially uniformly distribute the material of the preform along the longitudinal length prior to expansion of the preform against the mold cavity.
Once the pre-blowing phase is complete, the pre-blowing valve is closed and the blowing valve is opened, the blowing pressure is provided to the stretched preform. Upon completion of the blowing phase, the blowing valve is closed and the air-recovery valve can be opened. During the air-recovery phase, a portion of the blowing pressure can be recovered for later use. For example, the blowing pressure may be reused for the next pre-blowing phase. Finally, the exhaust valve is opened to exhaust the remaining pressure from the formed product.
Due to the cost associated with the air required for the pre-blowing and blowing phases, there have been numerous attempts to reduce the amount of air used and reduce the energy required to deliver the air to the preform. One prior art attempt is to use a single proportional valve for providing the air to the preform. Such an approach is outlined in WO/2011/154326, which is assigned on its face to the present applicants. Proportional valves are generally known in the art and can operate to open a port of the valve at virtually any point between fully open and fully closed. Therefore, rather than a simple on/off operation as in traditional valves, proportional valves are capable of maintaining an actuation state between fully on and fully off Although the approach proposed by the '326 application provides adequate proportional control in some situations, the use of a single proportional valve for the pre-blowing and the blowing pressure has serious drawbacks. For example, the proportional valve is often controlled with a proportional solenoid, which can increase the energy and cost associated with operation of the valve. Furthermore, the system proposed by the '326 application utilizes a single pressure supply, which needs to be at the higher blowing pressure, i.e., 40 bar (580 psi). With a single pressure supply, it is difficult, if not impossible, to recover any of the air at the end of a molding cycle. Consequently, the system fails to recycle any of the air resulting in excessive consumption of air.
The embodiments described below overcome these and other problems and an advance in the art is achieved. The embodiments described below provide a stretch blow molding system with a single blow mold valve in fluid communication with a low-pressure source and a high-pressure source. The low-pressure source is utilized during the pre-blowing phase while the high-pressure source can be utilized during the blowing phase.